ABSTRACT. We provide an assessment of the supraglacial water budget of a moulin basin on the western margin of the Greenland ice sheet for 15 days in August 2009. Meltwater production, the dominant input term to the 1.14 AE 0.06 km 2 basin, was determined from in situ ablation measurements.The dominant water-output terms from the basin, accounting for 52% and 48% of output, respectively, were moulin discharge and drainage into crevasses. Moulin discharge exhibits large diurnal variability (0.017-0.54 m 3 s -1) with a distinct late-afternoon peak at 16:45 local time. This lags peak meltwater production by $2.8 AE 4.2 hours. An Extreme Ice Survey time-lapse photography sequence complements the observations of moulin discharge. We infer, from in situ observations of moulin geometry, previously published borehole water heights and estimates of the temporal lag between meltwater production and observed local ice surface uplift ('jacking'), that the transfer of surface meltwater to the englacial water table via moulins is nearly instantaneous (<30 min). We employ a simple crevasse mass-balance model to demonstrate that crevasse drainage could significantly dampen the surface meltwater fluctuations reaching the englacial system in comparison to moulin discharge. Thus, unlike crevasses, moulins propagate meltwater pulses to the englacial system that are capable of overwhelming subglacial transmission capacity, resulting in enhanced basal sliding.

[1] A key mechanism for the rapid collapse of both the Larsen A and B Ice Shelves was meltwater-driven crevasse propagation. Basal crevasses, large-scale structural features within ice shelves, may have contributed to this mechanism in three important ways: i) the shelf surface deforms due to modified buoyancy and gravitational forces above the basal crevasse, creating >10 m deep compressional surface depressions where meltwater can collect, ii) bending stresses from the modified shape drive surface crevassing, with crevasses reaching 40 m in width, on the flanks of the basal-crevasseinduced trough and iii) the ice thickness is substantially reduced, thereby minimizing the propagation distance before a full-thickness rift is created. We examine a basal crevasse (4.5 km in length, $230 m in height), and the corresponding surface features, in the Cabinet Inlet sector of the Larsen C Ice Shelf using a combination of high-resolution (0.5 m) satellite imagery, kinematic GPS and in situ ground penetrating radar. We discuss how basal crevasses may have contributed to the breakup of the Larsen B Ice Shelf by directly controlling the location of meltwater ponding and highlight the presence of similar features on the Amery and Getz Ice Shelves with high-resolution imagery. Citation:

We identify a series of basal crevasses along a 31 km transect across the northern sector of the Larsen C ice shelf, Antarctica, using in situ ground-penetrating radar. The basal crevasses propagate from a region of multiple, shallow basal fractures to form widely spaced (0.5-2.0 km) but deeply incised (70-134 m) features. Surface troughs, observed in visible imagery, exist above the basal crevasses as the ice vertically shears to reach hydrostatic equilibrium, while widespread surface crevassing occurs along the crests and on the flanks of the undulations, primarily aligned with the topography. We suggest, based on the location of the surface crevasses and the along-flow evolution of the basal crevasses, that the former are induced by a bending stress created by gradients in hydrostatic forces. Using a linear elastic fracture mechanics model, we investigate the sensitivity of basal crevasse propagation to observed trends of ice-shelf thinning and acceleration. Basal crevasses are large-scale structural weaknesses that can both control meltwater ponding and induce surface crevassing. Together, these features may represent an important mechanism in both past and future ice-shelf disintegration events on the Antarctic Peninsula.

[1] Observations at Summit, Greenland suggest that the annual mean near-surface air temperature increased at 0.09 AE 0.01 C/a over the 1982-2011 climatology period. This rate of warming, six times the global average, places Summit in the 99th percentile of all globally observed warming trends over this period. The rate of warming at Summit is increasing over time. During the instrumental period , warming has been greatest in the winter season, although the implications of summer warming are more acute. The annual maximum elevation of the equilibrium line and dry snow line has risen at 44 and 35 m/a over the past 15 and 18 years, respectively. Extrapolation of this observed trend now suggests, with 95% confidence intervals, that the dry snow facies of the Greenland Ice Sheet will inevitably transition to percolation facies. There is a 50% probability of this transition occurring by 2025.